1. Field of the Invention
This invention relates generally to optical devices, and more particularly to a thermorefractive switch or attenuator utilizing temperature dependence of the refractive index of a waveguide core to effect switching or attenuation in an optical device.
2. Technical Background
Optical switching devices are known which utilize materials in which the refractive index of a polymer waveguide material can be controlled through various phenomena such as by a second order nonlinear electrooptic effect, a third order nonlinear optical Kerr effect, a thermooptic effect, or an acoustooptic effect. Polymers have been used in thermooptic switches because of their large negative variations in the index of refraction as a function of temperature (dn/dT), which are typically about xe2x88x923xc3x9710xe2x88x924Kxe2x88x921. Stated differently, polymer materials are useful in the fabrication of thermooptic switches because a relatively small temperature change can effect a relatively large change in the refractive index of the polymer.
Typical polymer based thermooptic switches use thin film electrical strip heaters in contact with a planar polymer waveguide. These heaters effect a change in the refractive index thus causing thermooptic switching to occur. Fabrication of such devices requires metal electrode deposition techniques and integration of electronics into the planar optical devices which increase the complexity and cost of the device. A further disadvantage is that most electrically heated thermooptical devices are limited to switching speeds of about one to about two milliseconds due to thermal diffusion time lag.
It has been known that thermooptic effects can be induced in polymers by a photothermal phenomena in which light absorbed by the polymer is converted into heat which causes a change in refractive index. The change in temperature in a material due to absorption at steady state can be approximated by the following equation:       Δ    ⁢          xe2x80x83        ⁢    T    =            α      ⁢              xe2x80x83            ⁢      P      ⁢              xe2x80x83            ⁢              τ        c                    π      ⁢              xe2x80x83            ⁢              r        2            ⁢      ρ      ⁢              xe2x80x83            ⁢      C      
where xcex1 is the absorption coefficient, P is the steady state power, xcfx84C is the characteristic decay time after the power has been turned off, r is the spot size radius of the area of the material which is irradiated with light having steady state power P, xcfx81 is the density of the material, and C is the heat capacity of the material. By exploiting photothermal effects, active switches can be developed by inducing local refractive index changes due to a finite amount of localized absorption of light by the polymer. A known device utilizing a photothermal effect for optical switching comprises a substrate of light absorbent material, means defining a plurality of holes of pre-selected size through the substrate, the holes being defined through the substrate, and a liquid material disposed within the holes. The liquid material has an index of refraction which is substantially temperature dependent over a selected temperature range of operation for the device. A disadvantage with this device is that it uses a liquid material which could potentially escape from the device, rendering the device inoperative. Accordingly, a solid state device is preferable.
An optical switch comprising a waveguide having a polymeric waveguide core including a region containing molecules which absorb energy from a light source and thereby heat the core and change the refractive index of the core are described in the patent literature. The device includes a waveguide having an input region, a Y-branch which splits light entering the input region into the two separate branches, and a coupling region. One of the two branches includes a means for changing the temperature to cause a change in refractive index, which in turn results in a phase shift between the light propagated through each of the two branches. When the light enters the coupling region a predetermined transfer, or switching, of light occurs from one leg to the other, with the amount of the transfer depending upon the phase change.
The invention provides an optical switching device which is capable of achieving submillisecond switching or attenuation for regions that are irradiated on the order of the size of a single mode waveguide mode field diameter.
In accordance with an aspect of this invention, an optical switch is provided which includes an optical splitter having a waveguide defined by a core and a surrounding cladding, wherein the waveguide includes an input leg, and first and second branch legs. The branch legs and the input leg are joined at a junction wherein light may be propagated from the input leg through each of the branch legs. The first branch leg includes at least a region comprised of a material having an absorption coefficient at a switch wavelength that is higher than an absorption coefficient at a signal wavelength.
In accordance with another aspect of the invention, a variable optical attenuator is provided. The variable attenuator includes a waveguide defined by a core and a surrounding cladding, wherein the core includes a first input waveguide section, a second waveguide section branching from the first waveguide section at a first junction, a third waveguide section branching from the first waveguide section at the first junction, and a fourth output waveguide section joined to the second and third waveguide sections at a second junction. The second waveguide section includes a region comprising a material which changes refractive index when exposed to light at an attenuation wavelength, but which is unresponsive to light at a signal wavelength.